Running coiled tubing in open sea without using a marine riser or a workover riser imposes requirements on the operation of the vessel and the coiled tubing. Because of the limited mechanical strength of the coiled tubing and the subsea stack including the lubricator pipe it is imperative that the equipment be operated within certain predefined limits related to the structural capacities of the equipment. This implies that the following quantities need be controlled or monitored either directly or indirectly:
Top tension of CT (Coiled Tubing)
Declination of the CT when leaving the top injector at the vessel
Bending of the CT when entering the lubricator
Tension of CT when entering the lubricator
The means for keeping control of these quantities are the positioning of the vessel and the applied top tension in the coiled tubing. Three out of these four parameters are readily obtainable through direct measurements: top tension and declination at top injector; and indirect measurements: tension of CT at lubricator, derivable from the top tension and the apparent weight of CT.
Maintaining the structural integrity of the coiled tubing and the subsea stack is essential. The critical loads with respect to structural integrity are related to the entry of the coiled tubing into the lubricator, which will be close to vertical.
When the coiled tubing enters the lubricator it is locally restricted from freely changing shape as a response to the external loading. That is, the coiled tubing must satisfy the boundary conditions given by the entry into the lubricator pipe. Any deviation between the direction of the coiled tubing and the direction of the lubricator pipe will therefore introduce lateral forces between the coiled tubing and the lubricator pipe.
These lateral forces will locally induce bending moments in the coiled tubing. To avoid collapse caused by overbending of the coiled tubing and/or the lubricator pipe these loads must be controlled.
Positioning the vessel such that there is no local bending of the coiled tubing where it enters the lubricator pipe implies that the axial force in the coiled tubing is directed along the lubricator pipe.
Consequently there will be no lateral force acting on the lubricator pipe for this configuration of the coiled tubing. The vessel position that results in this coiled tubing configuration is the optimal one with respect to integrity of the coiled tubing and the subsea stack during operation.
Therefore, it is of importance to know the bending moment and declination of the coiled tubing as it enters the lubricator pipe. However, because the coiled tubing most of the time during operation is either being inserted into the well or being retracted, it is considered impractical to measure the declination or bending moment at lubricator entry directly on the coiled tubing itself.